Method for fabricating nitride semiconductor, method for fabricating nitride semiconductor device, and nitride semiconductor device

ABSTRACT

The method for fabricating a nitride semiconductor of the present invention includes the steps of: (1) growing a first semiconductor layer made of a first group III nitride over a substrate by supplying a-first group III source and a group V source containing nitrogen; and (2) growing a second semiconductor layer made of a second group III nitride on the first semiconductor layer by supplying a second group III source and a group V source containing nitrogen. At least one of the steps (1) and (2) includes the step of supplying a p-type dopant over the substrate, and an area near the interface between the first semiconductor layer and the second semiconductor layer is grown so that the density of the p-type dopant locally increases.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for fabricating anitride semiconductor in which the density of a p-type dopant ispositively increased, a method for fabricating a nitride semiconductordevice, and a nitride semiconductor device fabricated by this method.

[0002] Prior art techniques of doping a nitride semiconductor devicewith a p-type dopant, in particular, magnesium (Mg) will be described.

[0003] In the first prior art (Japanese Journal of Applied Physics, 38,L1012, 1999), a superlattice (SL) layer having a cycle of 36 nm isdisclosed for use as a p-type cladding layer paired with an n-typecladding layer to sandwich an active layer in the direction vertical toa substrate and confine light generated from the active layer. Eachcycle of the superlattice layer is composed of an aluminum galliumnitride (Al_(0.15)Ga_(0.85)N) layer having a thickness of 24 nm and agallium nitride (GaN) layer having a thickness of about 12 nm, forexample. In this disclosure, the cycle of the superlattice layer is inthe range of 9 nm to 100 nm.

[0004] Doping of the p-type cladding layer with magnesium (Mg) isperformed uniformly over the entire superlattice layer. There is anotherdisclosure reporting doping of either the AlGaN layers or the GaNlayers. In either case, doping is uniform in each layer of the AlGaNlayers and/or the GaN layers. This p-type cladding layer is formed on asubstrate in a following manner. That is, using decompressedmetal-organic vapor phase epitaxy (MOVPE) under a growth pressure of 300Torr (1 Torr=133.322 Pa), a buffer layer made of aluminum nitride (AlN)is grown on a sapphire substrate of which the principal plane is the Cplane at a substrate temperature of 400° C., and subsequently an undopedgallium nitride (GaN) layer having a thickness of 1 μm is grown on thebuffer layer at a raised temperature. The substrate temperature is thenraised to 1010° C., and the superlattice layer is grown.

[0005] By adopting the above method, strain occurs between the AlGaNlayer and the GaN layer, causing generation of an internal electricfield. This makes the acceptor level of Mg shallow and thus improves theactivation yield of the acceptor. Therefore, the p-type carrier density(hole density) increases, and this advantageously reduces the thresholdcurrent of the laser device.

[0006] In the second prior art (Japanese Laid-Open Patent PublicationNo. 8-97471), disclosed is a first contact layer made of highly dopedp-type GaN that is in contact with an electrode made of nickel (Ni). Thefirst contact layer has a thickness of 50 nm and a Mg density in therange of 1×10²⁰ cm⁻³ to 5×10²¹ cm⁻³. This prior art discusses that withthis construction, the contact resistance can be reduced, and also theoperating voltage of the device can be lowered by attaining a highcarrier density.

[0007] In the second prior art, if the first contact layer is doped withMg at an excessively high density, the hole density contrarily becomeslow. To overcome this problem, a second contact layer made of p-type GaNhaving a Mg density lower than the first contact layer is formed on thesurface of the first contact layer opposite to the electrode. Accordingto this prior art, the second contact layer is desirably doped with Mgat a density in the range of 1×10¹⁹ cm⁻³ to 5×10²⁰ cm⁻³ for the purposeof increasing the hole density.

[0008] The prior art techniques described above have the followingproblems. In the first prior art, the superlattice structure of thep-type cladding layer is yet insufficient in attaining low resistance.In the second prior art, although the upper portion of the p-typecontact layer is doped with the p-type dopant at a high density, thiscontrarily decreases the hole density.

[0009] In addition, the conventional doping techniques find difficultyin providing a steep impurity profile. In particular, when a p-type caplayer is formed on an active layer, for example, an especially steepimpurity profile is required for suppression of diffusion of a p-typedopant to the active layer.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is attaining low resistance ofa nitride semiconductor by increasing the p-type impurity density of thenitride semiconductor without increasing the doping amount and alsoproviding a steep p-type impurity profile.

[0011] To attain the above object, according to the present invention, afirst semiconductor layer made of a group III nitride is formed incontact with a second semiconductor layer made of a group III nitridedifferent in composition from that of the first semiconductor layer, sothat the density of a p-type dopant locally increases in an area nearthe heterojunction interface between the first and second semiconductorlayers, that is, so that segregation of the p-type dopant occurs.

[0012] The method for fabricating a nitride semiconductor of the presentinvention includes the steps of: (1) growing a first semiconductor layermade of a first group III nitride over a substrate by supplying a firstgroup III source and a group V source containing nitrogen; and (2)growing a second semiconductor layer made of a second group III nitrideon the first semiconductor layer by supplying a second group III sourceand a group V source containing nitrogen, wherein at least one of thesteps (1) and (2) includes the step of supplying a p-type dopant overthe substrate, and an area near the interface between the firstsemiconductor layer and the second semiconductor layer is grown so thatthe density of the p-type dopant locally increases.

[0013] According to the method for fabricating a nitride semiconductorof the present invention, by forming a layered structure of the firstsemiconductor layer and the second semiconductor layer, the density ofthe p-type dopant in the layered structure increases compared with theconventional case. This makes it possible to attain low resistance, andalso attain a steep p-type impurity profile in which only the layeredstructure has a high impurity density.

[0014] In the method for fabricating a nitride semiconductor of thepresent invention, preferably, the first group III source containsgallium, and the second group III source contains aluminum or indium.This further ensures increase in the density of the p-type dopant in thelayered structure compared with the conventional case.

[0015] In the method for fabricating a nitride semiconductor of thepresent invention, preferably, the first group III source mainlycontains gallium, and the second group III source contains gallium andeither one of aluminum and indium. This further ensures increase in thedensity of the p-type dopant in the layered structure compared with theconventional case.

[0016] In the method for fabricating a nitride semiconductor of thepresent invention, when both the step (1) and the step (2) include thestep of supplying a p-type dopant, the supply amount of the p-typedopant is preferably roughly the same in the two steps. Even in thisuniform doping with the p-type dopant, it is possible to locallyincrease the density of the p-type dopant in an area near the interfacebetween the first and second semiconductor layers.

[0017] In the method for fabricating a nitride semiconductor of thepresent invention, the supply amount of the p-type dopant is preferablydifferent between the step (1) and the step (2). Even in this selectivedoping with the p-type dopant, it is possible to locally increase thedensity of the p-type dopant in an area near the interface between thefirst and second semiconductor layers.

[0018] In the method for fabricating a nitride semiconductor of thepresent invention, when the p-type dopant is supplied during the growthof the first semiconductor layer, the supply of the p-type dopant ispreferably started ahead of the growth of the first semiconductor layer.Likewise, when the p-type dopant is supplied during the growth of thesecond semiconductor layer, the supply of the p-type dopant ispreferably started ahead of the growth of the second semiconductorlayer. By this advanced supply of the p-type dopant, the p-type dopantthat is to be introduced into the semiconductor layer under growth canreach the growth surface of the semiconductor layer without delay. Thisensures attainment of a steep impurity profile.

[0019] In the method for fabricating a nitride semiconductor of thepresent invention, the peak of the density of the p-type dopant ispreferably located in the second semiconductor layer.

[0020] In the method for fabricating a nitride semiconductor of thepresent invention, preferably, the second group III source contains aplurality of group III elements, and the peak position of the density ofthe element having a smaller mole fraction among the plurality of groupIII elements is different from the peak position of the density of thep-type dopant.

[0021] In the method for fabricating a nitride semiconductor of thepresent invention, the density of the p-type dopant is preferably about3×10¹⁹ cm⁻³ or less. This makes it possible to increase the effectiveacceptor density of the p-type dopant.

[0022] In the method for fabricating a nitride semiconductor of thepresent invention, the thickness of the second semiconductor layer ispreferably about 1.5 nm or more. By this setting, it is possible tolocate the peak position of the p-type dopant inside the secondsemiconductor layer.

[0023] The method for fabricating a nitride semiconductor device of thepresent invention includes the steps of: (1) growing an active layermade of a first nitride semiconductor on a substrate; (2) growing ap-type cap layer made of a second nitride semiconductor on the activelayer for protecting the active layer; (3) growing a p-type claddinglayer made of a third nitride semiconductor on the p-type cap layer; and(4) growing a p-type contact layer made of: a fourth nitridesemiconductor on the p-type cladding layer, wherein at least one of thesteps (2), (3), and (4) includes the steps of: growing one layer made ofa first group III nitride by supplying a first group III source and agroup V source containing nitrogen; and growing another layer made of asecond group III nitride on the one layer by supplying a second groupIII source and a group V source containing nitrogen, wherein at leastone of the step of growing one layer and the step of growing anotherlayer includes the step of supplying a p-type dopant to the substrate,and an area near the interface between the one layer and the anotherlayer is grown so that the density of the p-type dopant locallyincreases.

[0024] According to the method for fabricating a nitride semiconductordevice of the present invention, the method for fabricating a nitridesemiconductor of the present invention is employed for formation of atleast one of the p-type cap layer, the p-type cladding layer, and thep-type contact layer of the nitride semiconductor device. This makes itpossible to attain low resistance and a steep impurity profile for atleast one of the p-type cap layer, the p-type cladding layer, and thep-type contact layer.

[0025] In the method for fabricating a nitride semiconductor device ofthe present invention, preferably, the first group III source containsgallium, and the second group III source contains aluminum or indium.

[0026] In the method for fabricating a nitride semiconductor device ofthe present invention, the supply of the p-type dopant is preferablystarted before the growth of the one layer or the another layer.

[0027] In the method for fabricating a nitride semiconductor device ofthe present invention, the density of the p-type dopant in the p-typecap layer or the p-type cladding layer is preferably about 3×10¹⁹ cm⁻³or less.

[0028] In the method for fabricating a nitride semiconductor device ofthe present invention, the thickness of the another layer is preferablyabout 1.5 nm or more.

[0029] In the method for fabricating a nitride semiconductor device ofthe present invention, preferably, the p-type contact layer containsindium, and the density of the p-type dopant in the p-type contact layergradually decreases as the position is deeper from the surface of thep-type contact layer, and is about 3×10¹⁹ cm⁻³ or more at a positionabout 10 nm deep from the top surface.

[0030] The nitride semiconductor device of the present inventionincludes: an active layer made of a first nitride semiconductor formedon a substrate; a p-type cap layer made of a second nitridesemiconductor formed on the active layer; a p-type cladding layer madeof a third nitride semiconductor formed on the p-type cap layer; and ap-type contact layer made of a fourth nitride semiconductor formed onthe p-type cladding layer, wherein at least one of the p-type cap layer,the p-type cladding layer, and the p-type contact layer has a multilayerstructure of one layer made of a first group III nitride and anotherlayer made of a second group III nitride different from the first groupIII nitride formed on the one layer, and the density of the p-typedopant locally increases in an area of the another layer near theinterface with the one layer.

[0031] In the nitride semiconductor device of the present invention,preferably, the first group III source contains gallium, and the secondgroup III source contains aluminum or indium. This makes it possible toattain a semiconductor layer device capable of oscillatingshort-wavelength laser light such as violet light.

[0032] In the nitride semiconductor device of the present invention, thedensity of the p-type dopant in the p-type cap layer or the p-typecladding layer is preferably about 3×10¹⁹ cm⁻³ or less.

[0033] In the nitride semiconductor device of the present invention, thethickness of the another semiconductor layer is preferably about 1.5 nmor more.

[0034] In the nitride semiconductor device of the present invention,preferably, the another layer of the p-type contact layer containsindium, and the density of the p-type dopant in the p-type contact layergradually decreases as the position is deeper from the surface of thep-type contact layer, and is about 3×10¹⁹ cm⁻³ or more at a positionabout 10 nm deep from the top surface.

[0035] The 61st Annual Meeting Digest 3a-Y-30, September, 2000 of TheJapan Society of Applied Physics describes a strained-layer superlattice(SLS) structure with each cycle of 5 nm composed of anAl_(0.16)Ga_(0.84)N layer having a thickness of 2.5 nm and a GaN layerhaving a thickness of 2.5 nm. Both the AlGaN layers and the GaN layersare doped with Mg, a p-type dopant, uniformly at a density of 7×10¹⁹cm⁻³. From analysis by secondary ion mass spectrometry (SIMS), aphenomenon that Mg is selectively incorporated in the AlGaN layers asthe barrier layers is observed although the cycle is extremely short. Inthis case, however, because the resolution in the substrate depthdirection is not sufficient, it is impossible to determine whether ornot segregation of Mg has occurred in areas near the heterojunctioninterfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a structural cross-sectional view of a nitridesemiconductor laser device in an embodiment of the present invention.

[0037]FIG. 2A is a structural cross-sectional view of a p-typesuperlattice cap layer of the nitride semiconductor laser device in theembodiment of the present invention.

[0038]FIG. 2B is a structural cross-sectional view of a p-type firstcontact layer of the nitride semiconductor laser device in theembodiment of the present invention.

[0039]FIGS. 3A and 3B are graphs of impurity profiles by SIMS of thep-type superlattice cap layer of the nitride semiconductor laser devicein the embodiment of the present invention, obtained when adoptingselective doping (3A) and uniform doping (3B).

[0040]FIG. 4 is a graph of an impurity profile by SIMS of the p-typefirst contact layer of the nitride semiconductor laser device in theembodiment of the present invention.

[0041]FIGS. 5A and 5B are graphs of density profiles of the p-type firstcontact layer and the p-type second contact layer of a semiconductorlayer device in a modification of the embodiment of the presentinvention (5A) and a conventional semiconductor laser device (5B).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Hereinafter, a preferred embodiment of the present invention willbe described with reference to the accompanying drawings.

[0043]FIG. 1 shows a cross-sectional construction of a nitridesemiconductor laser device in an embodiment of the present invention.Hereinafter, the construction of the semiconductor laser device will bedescribed together with the fabrication process thereof. Metal-organicvapor phase epitaxy (MOVPE) is employed for crystal growth, and thegrowth pressure may be a decompressed pressure, the atmospheric pressure(1 atm), or a compressed pressure higher than the atmospheric pressure.Alternatively, the growth pressure may be changed to an appropriatepressure for each semiconductor layer. As a carrier gas for supplying amaterial gas to the substrate, at least an inert gas such as nitrogen orhydrogen is used.

[0044] Referring to FIG. 1, first, the growth temperature is set atabout 500° C., and ammonia (NH₃) as a group V source andtrimethylgallium (TMG) as a group III source are supplied to theprincipal plane of a substrate 11 made of sapphire, for example, to growa buffer layer 12 made of gallium nitride (GaN) having a thickness ofabout 20 nm on the principal plane of the substrate 11.

[0045] The substrate temperature is then raised to about 1020° C., andNH₃ and TMG are supplied to grow an underlying layer 13 made of GaNhaving a thickness of about 1 μm on the buffer layer 12. A resistpattern (not shown) is then formed on the underlying layer 13 byphotolithography. The resist pattern is composed of stripes having awidth of about 3 μm extending in parallel with each other with a spacingof about 12 μm between the adjacent stripes. Using the resist pattern asa mask, the underlying layer 13 is dry-etched, to form a plurality ofrecesses 13 a and a plurality of stripe-shaped convex portions 13 bbetween the adjacent recesses 13 a in the upper portion of theunderlying layer 13. Silicon nitride (SiN_(x)) is then deposited overthe entire surface of the underlying layer 13 with the recesses 13 a,including the resist pattern and the convex portions 13 b, by ECRsputtering, for example, to form a mask film 14. The portions of themask film 14 located on the convex portions 13 b are then removed bylifting off the resist pattern, to expose the top faces of the convexportions 13 b.

[0046] Thereafter, the growth temperature is set at about 1000° C., andNH₃ and TMG are supplied to the underlying layer 13, to grow a selectivegrowth layer 15 made of GaN having a thickness of about 3 μm by MOVPEagain, using the exposed faces of the convex portions 13 b of theunderlying layer 13 as seed crystal.

[0047] Subsequently, with the growth temperature set at about 1000° C.,NH₃, TMG, and an n-type dopant including silicon (Si), for example, aresupplied, to grow an n-type contact layer 16 made of n-type GaN having athickness of about 2 μm on the selective growth layer 15.

[0048] NH₃, TMG, trimethylaluminum (TMA), and an n-type dopant are thensupplied to grow an n-type cladding layer 17 made of n-type aluminumgallium nitride (Al_(0.07)Ga_(0.93)N) having a thickness of about 0.7 μmon the n-type contact layer 16.

[0049] NH₃, TMG, and an n-type dopant are then supplied, to grow ann-type optical guide layer 18 made of n-type GaN having a thickness ofabout 100 nm on the n-type cladding layer 17.

[0050] The growth temperature is then lowered to about 810° C., and bysupplying NH₃, TMG, and trimethylindium (TMI), a multiple quantum well(MQW) active layer 19 is formed on the n-type optical guide layer 18.The active layer 19 is composed of three cycles of semiconductor layers,and each cycle is composed of a well layer made of indium galliumnitride (In_(0.1)Ga_(0.9)N) having a thickness of about 3 nm and abarrier layer made of GaN having a thickness of about 6 nm on the welllayer. The supply of TMI is stopped during the growth of the barrierlayers.

[0051] While the growth temperature is raised again to about 1020° C.,NH₃, TMG, and bis(cyclopentadienyl)magnesium (Cp₂Mg) containingmagnesium as a p-type dopant are supplied to form a p-type superlatticecap layer 20 having a thickness of about 20 nm on the active layer 19.As shown in FIG. 2A, the superlattice cap layer 20 is composed of fourcycles of semiconductor layers, and each cycle is composed of a firstlayer (well layer) 20 a made of GaN having a thickness of about 2.5 nmand a second layer (barrier layer) 20 b made of Al_(0.3)Ga_(0.7)N havinga thickness of about 2.5 nm. TMA is additionally supplied as anothergroup III source during the growth of the second layers.

[0052] Subsequently, NH₃, TMG, and Cp₂Mg are supplied to grow a p-typeoptical guide layer 21 made of p-type GaN having a thickness of about100 nm on the p-type superlattice cap layer 20.

[0053] NH₃, TMG, and Cp₂Mg are again supplied to form a p-typesuperlattice cladding layer 22 having a thickness of about 0.7 μm on thep-type optical guide layer 21. The superlattice cladding layer 22 iscomposed of 140 cycles of semiconductor layers, and each cycle iscomposed of a first layer (well layer) made of GaN having a thickness ofabout 2.5 nm and a second layer (barrier layer) made ofAl_(0.14)Ga_(0.86)N having a thickness of about 2.5 nm. TMA isadditionally supplied as another group III source during the growth ofthe second layers.

[0054] Subsequently, NH₃, TMG, and Cp₂Mg are supplied to grow a p-typesecond contact layer 23 made of p-type GaN having a thickness of about100 nm on the p-type superlattice cladding layer 22.

[0055] NH₃, TMG, and Cp₂Mg are again supplied to form a p-type firstcontact layer 24 of a superlattice structure having a thickness of about10 nm to 12 nm on the p-type second contact layer 23. As shown in FIG.2B, the first contact layer 24 is composed of two cycles ofsemiconductor layers, and each cycle is composed of a first layer(barrier layer) 24 a made of GaN having a thickness of about 2.5 nm to 3nm and a second layer (well layer) 24 b made of In_(0.07)Ga_(0.93)Nhaving a thickness of about 2.5 nm to 3 nm. TMI is additionally suppliedas another group III source during the growth of the second layers.

[0056] Thus, the epitaxial layers constituting the semiconductor laserdevice are formed.

[0057] In this embodiment, the superlattice layers are doped with thep-type dopant uniformly throughout the growth of the first and secondlayers. That is, the supply amount of Cp₂Mg is constant. Alternatively,the supply amount of the p-type dopant may be different between thefirst and second layers, adopting so-called modulation doping.Otherwise, so-called selective doping may be adopted, where only eitherthe first layers or the second layers are doped. In these cases adoptingnon-uniform doping, also, the p-type dopant density increases in an areaof each aluminum- or indium-containing layer located near the interfaceon the substrate side (the surface at which growth started). Inselective doping, doping is preferably performed for the aluminum- orindium-containing layers.

[0058] Thereafter, the resultant epitaxial layers are etched using amask covering a stripe-shaped resonator formation region so that then-type contact layer 16 is exposed. Further, the p-type superlatticecladding layer 22, the p-type second contact layer 23, and the p-typefirst contact layer 24 in the resonator formation region formed by theabove etching are etched to form a ridge 30 serving as a currentinjection region in the upper portion of the resonator formation region.The ridge 30 is preferably placed at a position deviated from areasright above the convex portions 13 b of the underlying layer 13. By thisplacement, the device is less influenced by crystal dislocation that mayoccur in the selective growth layer 15, and this enables the current toflow in areas of the MQW active layer 19 and the like excellent incrystal quality.

[0059] Subsequently, a mask is formed to cover an electrode contactportion of the top surface of the p-type first contact layer 24 and anelectrode contact portion of the top surface of the n-type contact layer16. Under this mask, silicon dioxide (SiO₂) is deposited on the ridge 30and the exposed surface of the resonator formation region by CVD or thelike, to form a protection insulating film 25. The stripe width of theridge 30 is about 3 μm to 5 μm.

[0060] A p-side electrode 26 made of a multilayer structure of nickel(Ni) and gold (Au) is then formed by evaporation, for example, to fillthe opening of the protection insulating film 25 on the ridge 30 andalso cover the p-sides of the ridge 30. An n-side electrode 27 made of amultilayer structure of titanium (Ti) and aluminum (Al) is then formedby evaporation, for example, to fill the opening of the protectioninsulating film 25 on the n-type contact layer 16.

[0061] In this embodiment, the p-type superlattice cap layer 20, thep-type superlattice cladding layer 22, and the p-type first contactlayer 24 have a superlattice structure to attain low resistance of thep-type semiconductors and a steep impurity profile of the p-type dopant.Alternatively, at least one layer of the above three layers may have asuperlattice structure according to the present invention.

[0062] In the thus-fabricated semiconductor laser device, when a voltageis applied between the p-side electrode 26 and the n-side electrode 27,holes are injected from the p-side electrode 26 while electrons areinjected from the n-side electrode 27, toward the MQW active layer 19,resulting in electron-hole recombination at the MQW active layer 19. Bythis recombination, optical gains are generated and thus laseroscillation at a wavelength of about 404 nm occurs.

[0063] Hereinafter, the effectiveness of the superlattice structureaccording to the present invention used for the p-type superlattice caplayer 20, the p-type superlattice cladding layer 22, and the p-typefirst contact layer 24 will be described.

[0064] First, the p-type superlattice cap layer 20 and the p-typesuperlattice cladding layer 22 will be described. FIG. 3A shows animpurity profile in the substrate depth direction as the results ofmeasurement by SIMS of Mg contained in the p-type superlattice cap layer20 having four cycles each composed of the first layer 20 a made of GaNand the second layer 20 b made of Al_(0.3)Ga_(0.7)N. The growth of thesemiconductor layers proceeds from the right side toward the left sideas is viewed from the figure along the x-axis of the graph. Theselective doping was adopted in the illustrated example, where Cp₂Mgcontaining a p-type dopant was supplied only to the Al-containing secondlayers 20 b.

[0065] As shown in FIG. 3A, the Mg density increases in areas of theAl-containing second layers 20 b near the interfaces with the Al-freefirst layers 20 a on the substrate side. This indicates that selectivedoping of Mg for the Al-containing second layers 20 b is realized. Inthis way, a Mg density gradient is established in the superlatticestructure, where the density gradient gradually decreases toward the topsurface of the resultant substrate in each of the second layers 20 bmade of AlGaN. Moreover, as shown in FIG. 3A, as the lowest Mg densityof the second layers 20 b, a value in the same level as the impuritydensity obtained in the conventional method is secured.

[0066] Thus, the present inventors found the phenomenon that only byforming a heterojunction interface, a Mg density gradient is establishednear the interface although the supply amount of Cp₂Mg per unit time isconstant.

[0067] It has been confirmed that the peak of the Mg density of each ofthe second layers 20 b is at a position apart by about 1.5 nm from theinterface with the adjacent first layer 20 a on the substrate side. Inview of this, the thickness of the second layer 20 b is preferably atleast 1.5 nm so as to ensure that the peak of the impurity density islocated inside the second layer 20 b.

[0068] In this embodiment, the doping of Mg as the p-type dopant ispreferably started at the growth of a semiconductor layer located in aregion deeper than the p-type superlattice cap layer 20, that is, anunderlying semiconductor layer. The reason is as follows. If the Mgdoping is started simultaneously with the growth of the p-typesuperlattice cap layer 20, the material source of Mg, Cp₂Mg, fails to beimmediately supplied to the substrate, causing delay of the doping. Thismakes it difficult to attain a desired impurity profile.

[0069]FIG. 3B shows an impurity profile as measured by SIMS observedwhen both the first layers 20 a and the second layers 20 b are uniformlydoped with a p-type dopant. As shown in FIG. 3B, in spite of the uniformdoping, the density of Mg as the p-type dopant increases in areas of thesecond layers 20 b near the interfaces with the first layers 20 a on thesubstrate side, exhibiting segregation near the interfaces. Note thatthe results shown in FIG. 3B were obtained using a superlatticesemiconductor layer for measurement, which has eight cycles eachcomposed of the first layer 20 a and the second layer 20 b.

[0070] The phenomenon that the density of a p-type dopant increases inan area of each Al-containing second layer near the interface with theAl-free first layer on the substrate side was also confirmed for thep-type superlattice cladding layer 22.

[0071] Next, the effectiveness of the p-type first contact layer 24having the superlattice structure will be described.

[0072]FIG. 4 shows an impurity profile in the substrate depth directionas the results of measurement by SIMS of Mg contained in the p-typefirst contact layer 24 having the superlattice structure composed of thefirst layers 24 a made of GaN and the second layers 24 b made ofIn_(0.07)Ga_(0.93)N. From FIG. 4, it is observed that the density of Mgas the p-type dopant increases in areas of the second layers 24 b nearthe interfaces with the first layers 24 a on the substrate side,exhibiting segregation near the interfaces. This greatly increases theeffective acceptor density. Note that the results shown in FIG. 4 wereobtained using a superlattice semiconductor layer for measurement, whichhas four cycles each composed of the first layer 24 a and the secondlayer 24 b. (Modification)

[0073] Hereinafter, a modification of the embodiment of the presentinvention described above will be described with reference to therelevant drawings. The first contact layer 24 in this modification iscomposed of a single layer made of p-type InGaN having a thickness ofabout 10 nm.

[0074]FIG. 5A shows density profiles in the substrate depth direction ofthe Mg density of the p-type first contact layer made of a single layerand the p-type second contact layer measured by SIMS, and the effectiveacceptor density (Na-Nd) measured at various frequencies by acapacitance-voltage (C-V) measurement method.

[0075] As shown in FIG. 5A, when a single-layer p-type InGaN layer isused for the p-type first contact layer, the Mg density in an area ofthe p-type first contact layer near the interface with the p-type secondcontact layer sharply increases in a convex shape even when the uniformdoping is adopted where the supply amount of Cp₂Mg is constant.Moreover, the Mg density sharply increases in an area of the p-typefirst contact layer within about 5 nm from the surface thereof. Thus,the effective acceptor density increases greatly in the area of thep-type first contact layer within about 5 nm from the surface and alsoincreases a little in an area thereof over the range of 5 nm to 10 nmdeep from the surface, compared with the conventional method, exhibitingimprovement from the conventional method.

[0076] For comparison, FIG. 5B shows density profiles in the substratedepth direction of the Mg density of a p-type first contact layer madeof p⁺-GaN having a thickness of about 10 nm and a p-type second contactlayer made of p-GaN formed by a conventional method, and the effectiveacceptor density.

[0077] The layers shown in FIG. 5B were formed in the following process.The supply amount of Cp₂Mg was set at about 0.04 L/min (0° C., 1 atm)during the growth of the p-type second contact layer, and increasedabout seven times to about 0.27 L/min (0° C., 1 atm) during the growthof the p-type first contact layer. Despite this increase in supplyamount, the Mg density shows no signs of sharp increase in the area ofthe p-type first contact layer near the interface with the p-type secondcontact layer, that is, the area over the range of 5 nm to 10 nm deepfrom the top of the resultant substrate, but only shows monotonousincrease. The Mg density sharply increases in the area of the p-typefirst contact layer within 5 nm from the top surface. Thus, theeffective acceptor density increases only in the area of the p-typefirst contact layer within 5 nm from the top surface.

[0078] Therefore, in the conventional method where the supply amount ofCp₂Mg is increased only during the growth of the p-type first contactlayer, it is not possible to sharply increase the density of Mg as thedopant for the p-type first contact layer in the area of the p-typefirst contact layer near the interface with the p-type second contactlayer.

[0079] The present inventors found the following. If the Mg dopingdensity exceeds 3×10¹⁹ cm⁻³ in any of a GaN layer, an InGaN layer, andan AlGaN layer, Mg fails to substitute for original lattice positions ofgallium, nitrogen, and the like in the semiconductor crystal, and thusthe activation yield of Mg as the acceptor decreases.

[0080] In consideration of the above, the Mg doping densities of thep-type superlattice cap layer 20 and the p-type superlattice claddinglayer 22 are preferably set at 3×10¹⁹ cm⁻³ or less so as to avoid lossof carriers injected into the laser device in non-radiative centers suchas defects generated due to decrease in the activation yield of theacceptor.

[0081] As for the p-type first contact layer, there is no upper limit ofthe Mg density because it is more important to reduce the contactresistance against a metal electrode material.

[0082] The effect of increasing the density of a p-type dopant at theheterojunction interface works for the group III nitride semiconductorsas a whole. Therefore, the group III elements used are not limited toaluminum, gallium, and indium, but the heterojunction interface may beestablished using a semiconductor of boron nitride (BN), for example.

[0083] The p-type dopant is not limited to magnesium (Mg), but zinc(Zn), calcium (Ca), or the like may be used.

[0084] The semiconductor laser device in this embodiment includes theselective growth layer 15 grown on the stripe-shaped convex portions 13b formed between the recesses 13 a of the underlying layer 13 by anepitaxial lateral overgrowth (ELOG) method. The underlying layer 13 andthe selective growth layer 15 are not essential for the presentinvention. However, by adopting the ELOG method, considerably goodcrystallinity is obtained for the n-type contact layer 16 and thesubsequent semiconductor layers grown on the selective growth layer 15.

[0085] The method for growing a nitride semiconductor is not limited toMOVPE, but any methods capable of growing a nitride semiconductor, suchas hydride vapor phase epitaxy (H-VPE) and molecular beam epitaxy (MBE),may be employed.

What is claimed is:
 1. A method for fabricating a nitride semiconductor,comprising the steps of: (1) growing a first semiconductor layer made ofa first group III nitride over a substrate by supplying a first groupIII source and a group V source containing nitrogen; and (2) growing asecond semiconductor layer made of a second group In nitride on thefirst semiconductor layer by supplying a second group III source and agroup V source containing nitrogen, wherein at least one of the steps(1) and (2) includes the step of supplying a p-type dopant over thesubstrate, and an area near the interface between the firstsemiconductor layer and the second semiconductor layer is grown so thatthe density of the p-type dopant locally increases.
 2. The method ofclaim 1, wherein the first group III source contains gallium, and thesecond group III source contains aluminum or indium.
 3. The method ofclaim 1, wherein the first group III source mainly contains gallium, andthe second group III source contains gallium and either one of aluminumand indium.
 4. The method of claim 1, wherein when both the step (1) andthe step (2) include the step of supplying a p-type dopant, the supplyamount of the p-type dopant is roughly the same in the two steps.
 5. Themethod of claim 1, wherein the supply amount of the p-type dopant isdifferent between the step (1) and the step (2).
 6. The method of claim1, wherein when the p-type dopant is supplied during the growth of thefirst semiconductor layer, the supply of the p-type dopant is startedahead of the growth of the first semiconductor layer.
 7. The method ofclaim 1, wherein when the p-type dopant is supplied during the growth ofthe second semiconductor layer, the supply of the p-type dopant isstarted ahead of the growth of the second semiconductor layer.
 8. Themethod of claim 1, wherein the peak of the density of the p-type dopantis located in the second semiconductor layer.
 9. The method of claim 1,wherein the second group III source contains a plurality of group IIIelements, and the peak position of the density of the element having asmaller mole fraction among the plurality of group III elements isdifferent from the peak position of the density of the p-type dopant.10. The method of claim 1, wherein the density of the p-type dopant isabout 3×10¹⁹ cm⁻³ or less.
 11. The method of claim 1, wherein thethickness of the second semiconductor layer is about 1.5 nm or more. 12.A method for fabricating a nitride semiconductor device comprising thesteps of: (1) growing an active layer made of a first nitridesemiconductor on a substrate; (2) growing a p-type cap layer made of asecond nitride semiconductor on the active layer for protecting theactive layer; (3) growing a p-type cladding layer made of a thirdnitride semiconductor on the p-type cap layer; and (4) growing a p-typecontact layer lade of a fourth nitride semiconductor on the p-typecladding layer, wherein at least one of the steps (2), (3), and (4)includes the steps of: growing one layer made of a first group IIInitride by supplying a first group III source and a group V sourcecontaining nitrogen; and growing another layer made of a second groupIII nitride on the one layer by supplying a second group III source anda group V source containing nitrogen, wherein at least one of the stepof growing one layer and the step of growing another layer includes thestep of supplying a p-type dopant to the substrate, and an area near theinterface between the one layer and the another layer is grown so thatthe density of the p-type dopant locally increases.
 13. The method ofclaim 12, wherein the first group III source contains gallium, and thesecond group III source contains aluminum or indium.
 14. The method ofclaim 12, wherein the supply of the p-type dopant is started before thegrowth of the one layer or the another layer.
 15. The method of claim12, wherein the density of the p-type dopant in the p-type cap layer orthe p-type cladding layer is about 3×10¹⁹ cm⁻³ or less.
 16. The methodof claim 12, wherein the thickness of the another layer is about 1.5 nmor more.
 17. The method of claim 12, wherein the p-type contact layercontains indium, and the density of the p-type dopant in the p-typecontact layer gradually decreases as the position is deeper from thesurface of the p-type contact layer, and is about 3×10¹⁹ cm⁻³ or more ata position about 10 nm deep from the top surface.